A zoom lens having four lens groups such as in Japanese Laid Open Patent Application No. H5-297276 is known. The first lens group, in order from the enlarging side, is of negative refractive power and has a fixed position. The second lens group is of positive refractive power and moves in order to provide a zoom function. The third lens group is of negative refractive power and moves in order to correct for shifting of the image plane which otherwise would occur with zooming. The fourth lens group is of positive refractive power and is fixed in position. This type of zoom lens is often used with an image-sensing device, such as a small-sized CCD array, and so on. However, if one attempts to use such a lens for forming a large-sized image without modification in design of such a lens, the size of the lens that is needed becomes excessive. When this type of zoom lens is used as a projection lens, which requires a low value of distortion aberration, many zoom lenses of this type are unsuitable because the distortion is excessive.
When used for a projection lens with a liquid crystal display, it is desirable that the reducing side of the zoom lens be configured as an abbreviated telecentric optical system, in order that the light illuminating the liquid crystal display be normal to the display surfaces. However, many of the prior art devices lack those characteristics. In addition, few such zoom lenses have a sufficiently large back focus to allow room for insertion of an optical component for color separation or color synthesis between the lens system and the image surface.
In order to resolve such problems, a zoom lens as described in Japanese Laid Open Patent Application No. H10-268193 has been proposed which is constructed of five lens groups. The first lens group G.sub.1, in order from the enlarging side, has negative refractive power and is fixed in position. The second lens group G.sub.2 and the third lens group G.sub.3 are each of positive refractive power. The fourth lens group G.sub.4 has negative refractive power. The second through fourth lens groups G.sub.2 -G.sub.4 move in order to accomplish zooming and to simultaneously correct for shifting of the image plane which otherwise would accompany zooming. The fifth lens group G.sub.5 has positive refractive power and is fixed in position. In addition, certain specified conditions are satisfied.
However, concerning many newer liquid crystal projectors, a micro lens is attached to the front surface of the liquid crystal element. The micro lens widens the angle over which light is emitted, and efficiently absorbs diffracted light otherwise produced and reflected when ambient light is incident on the liquid crystal element.
A zoom lens that provides a bright image is thus required in this type of projector. The above-mentioned Japanese Laid Open Patent Application No.H10-268193 is known for its low F.sub.NO, of approximately 2.5. Nevertheless, there is demand for a zoom lens that is compact and yet has a sufficient back focus to accommodate color synthesizing components on its reducing side, has well-corrected aberrations so as to produce a high-quality, bright image, and is telecentric on its reducing side so as to be suitable for use with a liquid crystal display.